Typically, fingerprint capturing devices (readers) do not produce a good quality image when the finger being imaged is wet or has moisture on it. This is due to fingerprint image artifacts that lead to a degradation of the captured image. Typically the artifacts due to a moist finger result in bridging in the image between adjacent ridges R of a moist finger.
FIG. 1 shows a conventional fingerprint image capturing device. As shown in FIG. 1, the finger 1 is placed on the upper surface 2 of an optically transparent prism 3 (a glass, or plastic platen). The finger 1 is illuminated by a light source (not shown) positioned below the prism 3. The light 5 from the light source is incident on the finger surface placed on the prism 3 at an angle of incidence Φi. The reflected light from the prism 3 is then detected by a light detector (not shown). The aperture of the lens establishes the detection of the reflected light rays 4. In other words, the angle between the optical axis of the light detector and the line perpendicular to the prism surface 2 decides which light rays reach the detector (observation angle).
The angle of incidence that provides an angle of refraction of 90-degrees is known as the critical angle. Light will undergo total internal reflection for any angle of incidence greater than the critical angle. The components of the fingerprint imaging system are typically arranged in such a way that the angle of observation is greater than the critical angle for the interface between the prism and the air between the finger and the prism. The illumination source is also positioned in such a way that the generated light rays include a range of incidence angles including the angle of observation. From Snell law, the angle of incidence is equal to the angle of reflectance Φr, the range of angles of reflectance also includes the angle of observation.
Typically, the value of the critical angle at the interface between two materials depends on the index of refraction of those materials, n1 and n2. The index of refraction for air is approximately 1. The critical angle for the platen/air interface, based on the actual index of refraction of the platen material, forms a lower bound on the angle of observation and a limit on the angles of incidence. As a result, the orientation of the illumination source is also limited by the critical angle. Typically, the angle of observation is selected to be a bit larger than the critical angle.
Therefore, there is a need for an improved apparatus for capturing a high quality image of a moist finger.